String Theory
What Is String Theory? In This Chapter u Introducing string theory u Introducing loop quantum gravity u Small means big u Where the theories stand Studying physics is sort of like eating an artichoke. You pull off the layers of reality and slowly get to the heart of it all. Over the centuries, physicists have been able to explain an ever-wider range of phenomena with ever- fewer laws, and they now seem to be zeroing in on the root essence of the natural world. String theory and alternative theories are the latest steps in this effort—and maybe its culmination. This book aims to explain why these theories are potentially so revolutionary, what eye-popping things they reveal, and what problems remain to be cracked. The Ultimate Symphony One of the joys of childhood for youngsters is to play “Stump Your Teacher.” It’s a game students can always win and wise teachers encourage. When the teacher says that everything is made up of atoms, the bright student asks, “So what are atoms made of?” When the teacher replies that they’re made of sub- atomic particles called protons, neutrons, and electrons, the student asks, “What are protons and neutrons made of?” As the teacher answers “Even tinier particles called quarks,” the student then wants to know, “What are quarks and electrons made of?” At that point, the student wins. Not even the greatest expert in the world can answer that question. It’s the frontier of human knowledge. String theory lets teachers win one more round of the game. It proposes that subatomic particles are sub-sub-subatomic strings. If we zoom in on the particles closely enough, what we usually think of as little billiard balls reveal themselves to be tiny loops or lengths of a more primitive material. These strings vibrate like miniature guitar strings, and each type of particle corresponds to a string playing a certain pitch—as though quarks were middle C, electrons were E flat, and the world around us were a sym- phony of unimaginable intricacy. String theory unites not only the types of particles, but also the ways they behave. Currently, physicists must make do with an uneasy “shotgun marriage” of two explanations for the behavior of matter. Most phenomena, such as electricity and magnetism, fit into the conceptual framework known as quantum theory. But gravity stubbornly refuses to go along. It falls under the rubric of Albert Einstein’s general theory of relativity. String Theory Definiton: String theory proposes that matter, force, space, and time are composed of tiny vibrating strings. It’s widely considered the leading candidate for a unified theory of physics, which boils down all the forces and types of matter to a single set of prin- ciples. Quantum theory describes the behavior of objects based on the assumption that matter and force come in indivisible units. The reason for this split is that gravity is special. Whenever an object exerts a force on another object, the force travels through the space between those objects. But gravity does more. It also warps space. Gravity is like a truck that doesn’t just drive down a road but also causes the road surface to buckle as it does so. To bring gravity into the quantum frame- work requires a theory that can handle this special feature, a quantum theory of gravity. Such a theory converts the shotgun mar- riage into a true union. Because of the connection between gravity and the shape of space, a quantum theory of gravity would also be a quantum theory of space. Space might be far more complex than we give it credit for, like a road that looks smooth and unbroken from a distance but cracked and gnarled when viewed up close. String theory gets the most attention but is by no means the only contender for a deep theory of nature. Whereas string theorists see gravity as the lost sheep and seek to bring it back into the particle flock, phys- icists who prefer the leading alternative, known as loop quantum gravity, see gravity as the sheep dog. To them, the special features of gravity demand special treatment. According to this theory, space consists of atoms—not ordinary atoms, but little chunks of space that can’t be subdivided into anything smaller. Although loop grav- ity doesn’t set out to explain all the particles of nature, some of its proponents think it might still explain ordinary particles as little bits of tangled space, like knots in a carpet. Astronomer Carl Sagan famously said that we are all made of starstuff—chemical elements created deep within stars. Both string theory and loop gravity suggest we are made of spacestuff. The curious student might press on and ask, “So what are strings or atoms of space made of?” Physicists can’t answer that yet. These things might turn out to be the truly fundamental building blocks of the world, or they might be an approximate way of describing a still-deeper level of reality. The student who wants to know will have to join the effort to find out. Big Things Come in Small Packages Both the strings of string theory and the space atoms of loop gravity are small— incredibly small. By most estimates, an atom is to one of them what the entire observ- able universe is to a human being. No conceivable microscope will ever take a picture of a wriggling string. But physicists don’t really care about the building blocks, per se. They’re after the principles that govern our world, and they zoom in on the micro- scopic level simply because that’s where the principles are laid bare. In this, physics is like any domain of life where the guiding principles seem distant to us. What does it matter that we live in a democracy, for example? We don’t vote very often, and even when we do, our individual participation hardly affects the outcome. The principles of democracy don’t put food on your table or play rhythm guitar in your band. But without them, you might not even have a table or a band. The same goes for the principles of physics. They set the basic parameters of our existence, starting with the fact that we exist at all. When scientists centuries ago conjectured that the world is made of atoms, many people thought those tiny scraps of matter were abstractions that are irrelevant to our lives. The technology of the day could never hope to observe them. Yet the nature of atoms is essential to everything we see and do. If the world weren’t built of atoms, chemical reactions would fail to operate and life would be impossible. Likewise, strings or something else that fulfills a similar role are essential to under- standing how the world is put together. Without them, space and time might not even exist. Objects would have no location and events would have no duration; our world would be a static, structureless mush. So although strings may be small, the principles they embody are anything but. Exactly what those principles are, physicists aren’t yet sure. What they do know is that the principles are going to be revolutionary. The reason is that unifying quantum theory and general relativity isn’t simply a matter of force-fitting a few equations together. It requires two profoundly different ways of looking at the world to be reconciled, a task that famously stymied Einstein himself and has challenged every physicist since his time. Each of these worldviews has its failings, but each also has an integrity to it. It’s not at all obvious how to fix their faults without wrecking their successes. It takes some new conceptual input, some novel idea that human beings never before realized or appreciated. The different approaches to unification have varying degrees of ambition, but in some way or another, the unified theory will cover every phenomenon known to physics. Because of its scope, the theory will go right to the foundations of physical reality, and it will probably be unlike anything science has ever seen. Physicists get around the limitations of both relativity and quantum theory by saying that some deeper theory will explain them. A fully unified theory won’t be able to pass the buck. The conditions required to make such a theory work are so stringent that only a single set of concepts might be able to satisfy them. There might be no other way for a uni- verse to hang together. String theory comes closer to achieving this goal than any other effort that physicists have ever made. It’s not there yet, and it may well turn out to be completely wrong, but what encourages string theorists is that if we work through what it takes for a string to vibrate, it can do so only under very specific conditions. An ordinary guitar string doesn’t encounter the same restrictions. It’s so large and floppy (by physics standards) that the counterintuitive aspects of relativity and quantum theory don’t come into play. For a miniature string, though, things get more complicated—which is good, because the restrictions on its behavior serve as an organizing principle of nature. In this way, string theory helps us make sense of a world that so often seems senseless. String Instruments New principles always reveal themselves grudgingly. Consider how Einstein’s theories of relativity came about. The nineteenth-century experiment that paved the way for his theories—by discovering that light moves at a constant speed, independent of the speed of whatever emits it—had a precision of about 1 part in 10,000. Later, Einstein’s ideas about gravity were borne out by the shift in a position of a star on a photograph by little more than a hundredth of a millimeter. In fact, there’s a sort of inverse relationship: the broader the conceptual revolution, the harder you have to hunt for it. After all, if the new principles were so obvious, people would have noticed them already. For string theory, loop gravity, or whatever other explanation emerges for the inner- most workings of nature, the predicament is acute. Relativity and quantum theory make predictions that agree with observations, some as precise as 11 decimal places. This empirical success makes finding a new theory all the more difficult. The answer may lie in strings, but strings are small and their direct effects are proportionately tiny. Just managing to combine relativity and quantum theory into a single theory is a step. Any theory that unites them inherits their observational successes. But physicists also seek distinctive predictions—ways that strings go beyond what we already know to reveal unanticipated aspects of the universe, something about the world that people had been blind to. It could be a deviation in the twelfth decimal place, or even further down the line. Not only must instruments have the precision to test such a prediction, they must have the discriminating power to distinguish it from a thousand confounding effects with no deeper significance. Or there could be some other subtle clue that was staring us in the face all along. In ordinary life, it pays to be attuned to subtle clues. The “broken windows” theory in sociology is an example. A broken window that hasn’t been fixed or graffiti that hasn’t been scrubbed away seems fairly minor on its own. But the fact that people don’t attend to these little things hints at deeper problems. The poet William Blake gave the canine version: A dog starv’d at his master’s gate Predicts the ruin of the state. For someone to let his pet go hungry, something must be very wrong in a society— maybe economic hardship or a cycle of violence. To the trained eye, a seemingly minor occurrence is a sign of a much broader question of principle. Many physicists have worried whether such clues even exist for string theory and the other proposed theories. But things have been looking up lately. A number of new sci- entific instruments are able to test aspects of unified theories. The best known is the Large Hadron Collider, the largest particle smasher ever built—in fact, the largest and priciest scientific instrument of any kind. The collider is looking for novel phenom- ena and, if string theorists’ most optimistic predictions are right, could create fleeting Any discoveries by these instruments will involve incredibly tiny effects: one particle in a billion that acts up or two particles that race neck-and-neck for billions of years across the universe only to arrive a millisecond apart. And even those teensy signals don’t get at the core of string theory or loop gravity. No feasible instrument has anywhere near the resolution we’d need to prove or disprove either theory for sure. But proof or disproof in science is seldom so clear-cut. A theory steadily accumulates points in its favor or points against, until physicists judge that their time is better spent on something else. The next few years could prove decisive in either solidifying string theory or knocking it out. The fact that string theory and other such theories are works in progress is what makes them so exciting. We are watching ideas come together before our very eyes. Every generation thinks it lives in a special time, but a quantum theory of gravity could be our era’s claim to specialness. If one of the proposed theories works out, it will be one of the things future generations remember about us.